Value documents, that are at risk of counterfeiting, are normally provided with various security features to increase their security, so that unauthorized persons cannot undetectably change or reproduce them.
In this regard, the use of luminescent pigments or dyes to detect the validity of documents is generally known within the art. The main type of luminescence used in security documents is photoluminescence, which consists in the emission of light by the luminescent pigment when this is illuminated with light of a different wavelength. Usually the emission of light is produced in the visible range of the electromagnetic spectrum (400 nm-700 nm), when the pigment is excited with ultraviolet radiation (UV). However, other excitation-emission intervals are possible.
Photoluminescence can usually be classified into fluorescence and phosphorescence, depending on the lifetime of the radiation emitted upon excitation with an “instantaneous” pulse of light. “Lifetime” of photoluminescence (or, hereafter, simply “luminescence”) can be defined as the inverse of the rate constant of the first order kinetic process through which the spontaneous deactivation of the luminescent electronic state takes place after its formation. When the deactivation occurs through a kinetic process more complex than a first order one, it is usual to estimate a mean photoluminescence lifetime (such as those described by E. R. Carraway, J. N. Demas, B. A. DeGraff and J. R. Bacon, Analytical Chemistry 1991, 63, 337 or E. R. Carraway, J. N. Demas and B. A. DeGraff, Analytical Chemistry 1991, 63, 332). A pigment or dye is said to be “fluorescent” when the emission of light extinguishes after a very short time, on the order of nanoseconds, after excitation of the pigment has stopped. However, a pigment is referred to as “phosphorescent” if the emission of light continues for longer, on the order of milliseconds or more. The term “luminescence” includes all these phenomena, regardless of the duration of the emission.
The use of ruthenium based pigments to secure value documents is known in the art. European patent application EP1616929 discloses luminescent pigments of the family of ruthenium(II) complexes with chelating heterocyclic ligands, for encoding or validating security documents or goods.
Secured articles are commonly exposed to light, changes in temperature, humidity and other environmental perturbations. Consequently, there exists a need for developing security markings with improved stability. Further, security pigments or dyes with more difficult to elucidate structures are also required in order to enhance protection against counterfeiting.